The present invention relates to magnetic direction sensing systems and particularly those for use in vehicles.
U.S. Pat. No. 4,953,305 discloses an electrical compass system for use in a vehicle in which a magnetic sensor, such as a flux-gate sensor, is employed in connection with a microprocessor to provide a digital display of the vehicle heading. The system described in the '305 patent provides continuous automatic calibration of the compass to correct for changes in the detected magnetic field due to the vehicle's magnetism and changes thereto, as well as provides a system which, in effect, filters out magnetic aberrations caused by travel within an area where severe electromagnetic interference may temporarily occur.
In addition to the external magnetic disturbance which may affect the accuracy of display of compass headings, changes in the vehicle's magnetic field also can cause erroneous displays. One such problem is the type encountered when the compass sensor must be mounted in the instrument panel and the vehicle's HVAC system is activated. The fan motor draws significant current and, as the fan speed is varied drawing different amounts of current, the magnetic field caused by the current supplied to the fan motor can adversely affect the accuracy of the compass display. In order to correct for such problem, the compensation system disclosed in U.S. patent application Ser. No. 297,699 filed on August 29, 1994, and entitled VEHICLE COMPASS CORRECTION CIRCUIT (U.S. Pat. No. 5,511,319 was invented. In this system, correction signals are provided to the compass circuit in response to sensed fan motor speed settings to continuously provide accurate heading information regardless of the fan speed setting.
Regardless of the mounting position of the compass sensor, the vehicle's magnetism can also be affected when, for example, a vehicle door is opened. Typically, when the vehicle is underway, the doors are always closed; however, when the vehicle stops and the driver or passenger opens a door, the change in vehicle magnetism due to the movement of the door from a closed to an open position frequently will cause the compass heading to change even though the vehicle is stationary. Although this does not cause a navigational problem for the vehicle operator since the vehicle is not in motion, it can be perceived as a compass error and the confidence in future compass readings when the car is underway diminished. Further, even if the vehicle operator does not perceive the change in vehicle direction as a compass breakdown, it is an annoyance to the vehicle operator and one which has been the subject of complaints to dealers.
Accordingly, it is desirable to correct for this condition and the system of the present invention provides an improved compass circuit and software to due so.